In electric circuits and electronic circuits, various amplifiers are used for amplifying electrical signals to a predetermined level. Among these amplifiers, amplifiers mainly used as the final-stage amplifier in a transmitter and having a large output power are referred to as power amplifiers.
In radio sets for wide-area communications, for example, the transmission output is increased, and hence this results in an increase in the output power of the power amplifier used for the transmitter of the radio set. Thus, electric power consumed by the power amplifier is considerably increased, and electric power consumed by the transmitter occupies a large part of the entire power. Therefore, in order to decrease the power consumption of the transmitter, to decrease power consumption of the power amplifier, i.e. to improve efficiency of the power amplifier is an important issue.
Commonly, in the transistor used in the amplifier, its efficiency is excellent near saturation compared with the case in which the output signal level is low, and the efficiency is referred to as saturation effects. On the other hand, in radio communications in these years, in order to improve frequency usage efficiency, advanced digital modulation methods, such as orthogonal frequency-division multiplexing (OFDM), tend to be adopted. However, signals subjected to such modulation methods have a considerably large peak to average power ratio (PAPR). In the power amplifier, in order to keep the distortion of signal waveforms small, it is necessary to take into account of the maximum electric power of signals, regarding the saturated electric power of the transistor for use. Consequently, the power amplifier is operated with average power far less than saturated electric power, and the efficiency is prone to be much poorer than the efficiency near the saturated power.
As methods of solving the problems and embodying highly efficient power amplifiers, various methods are invented. Among these methods, the Doherty amplifier is increasingly adopted as a power amplifier suitable for improving the efficiency of signals with a large PAPR.
In these years, at the base stations of mobile telephones, various frequency bands are used from a 700 MHz band to a 2 GHz band. It is desired that one device be enabled to handle a plurality of frequency bands. Therefore, embodying wide-band enabled devices is an important issue. In digital terrestrial broadcasting, channels are allocated to the ultra high frequency (UHF) band from 400 to 800 MHz bands. When a transmitter that is enabled to handle all the channels in these frequency bands is to be embodied by one model, this is effective on inventory management, for example, and hence it is expected to embody wide-band enabled transmitters. In order that in transmitters for both of the base stations of mobile telephones and digital terrestrial broadcasting, signals subjected to advanced digital modulation are used and the signals are transmitted at high output, power amplifiers for use in these transmitters are also similarly needed to handle wide frequency bands.
In order to enable the Doherty amplifier to handle wide frequency bands, Japanese Unexamined Patent Application Publication No. 2012-29239 (Patent Literature 1), for example, discloses the following. For example, a carrier amplifier and a peak amplifier, which have an output impedance of five ohms, are divided into ten. Thus, the output impedance of the carrier amplifier and the peak amplifier, which are divided, is 50 ohms, and a quarter-wavelength phase line using a line having a characteristic impedance of 50 ohms can be formed. With the use of a plurality of the Doherty circuits, the outputs of the Doherty circuits are coupled with a coupler, and a Doherty amplifier is formed.